The present invention relates to a process for producing molten iron and/or ferroalloys from a metalliferous feed material, such as ores, partly reduced ores, and metal-containing waste streams, in a metallurgical vessel containing a molten bath.
The present invention relates particularly to a molten bath-based direct smelting process for producing molten iron and/or ferroalloys from a metalliferous feed material.
The term xe2x80x9cdirect smelting processxe2x80x9d is understood to mean a process that produces a molten metal (which term includes alloys), in this case iron and/or ferroalloys, from a metalliferous feed material.
The present invention relates more particularly to a molten bath-based direct smelting process which relies on a molten metal layer as a smelting medium and is generally referred to as the HIsmelt process.
In general terms, the HIsmelt process includes the steps of:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a direct smelting vessel;
(b) injecting metalliferous feed material and solid carbonaceous material into the metal layer via a plurality of lances/tuyeres;
(c) smelting metalliferous feed material to metal in the metal layer;
(d) causing molten material to be projected as splashes, droplets, and streams into a space above a nominal quiescent surface of the molten bath to form a transition zone; and
(e) injecting an oxygen-containing gas into the vessel via one or more than one lance/tuyere to post-combust reaction gases released from the molten bath, whereby the ascending and thereafter descending splashes, droplets and streams of molten material in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via the side walls in contact with the transition zone.
A preferred form of the HIsmelt process is characterized by forming the transition zone by injecting carrier gas, metalliferous feed material, solid carbonaceous material and optionally fluxes into the bath through lances that extend downwardly and inwardly through side walls of the vessel so that the carrier gas and the solid material penetrate the metal layer and cause molten material to be projected from the bath.
This form of the HIsmelt process is an improvement over earlier forms of the process which form the transition zone by bottom injection of carrier gas and solid carbonaceous material through tuyeres into the bath which causes droplets and splashes and streams of molten material to be projected from the bath.
The applicant has carried out extensive pilot plant work on operating the HIsmelt process with continuous discharge of molten iron and periodic tapping of molten slag from the direct smelting vessel and has made a series of significant findings in relation to the process.
One of the findings, which is the subject of the present invention, is that the pressure in the direct smelting vessel is an effective means of controlling the level of molten metal in the vessel. This finding is applicable particularly although by no means exclusively to direct smelting processes which discharge molten metal continuously and tap molten slag periodically.
In general terms, the present invention is a direct smelting process for producing molten iron and/or ferroalloys from a metalliferous feed material which includes the steps of:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a direct smelting vessel;
(b) supplying metalliferous feed material, carbonaceous material and fluxes into the vessel;
(c) smelting metalliferous feed material to molten iron in the molten bath;
(d) injecting an oxygen-containing gas into the vessel to post-combust gases generated in the process;
(e) continuously tapping molten metal from the vessel;
(f) periodically tapping molten slag from the vessel;
and which is characterised by controlling the level of molten metal in the vessel by adjusting the pressure in the vessel.
Preferably the process includes controlling the level of molten metal in the vessel by the steps of:
(i) increasing the pressure in the vessel at any time during a slag tap and up to 15 minutes after completing the slag tap to a pre-determined pressure P1 to compensate for an increase in metal height as a consequence of tapping slag from the vessel; and
(ii) after the vessel pressure reaches pressure P1, adjusting the pressure so that the pressure is a lower pressure P2 at the next slag tap to compensate for the effect of increasing slag inventory on metal height during this period.
Preferably the pressure increase in step (i) is at least 5 kPa.
Preferably step (i) includes increasing the pressure in the vessel at any time during the slap tap and up to 10 minutes after completing the slag tap.
Preferably step (i) includes increasing the pressure in the vessel only during the period of the slag tap.
The pressure may be increased in step (i) in a series of steps or continuously.
Preferably step (i) includes increasing the pressure in the vessel in a series of steps.
The pressure adjustment step (ii) may include decreasing the pressure in a series of steps or continuously.
Preferably adjustment step (ii) includes decreasing the pressure in a series of steps.
Preferably the time interval between pressure reduction steps is 20-30 minutes.
It is noted that within the above-described framework of decreasing pressure from pressure P1 to pressure P2 there may be short term perturbations during which there are one or more pressure changes against the established trend of reducing pressure to pressure P2. For example, in a situation where the vessel includes a forehearth for tapping molten metal, there may be a need between slag taps to reduce the vessel pressure to a pressure below P2 for a short period of time to allow the metal level in the vessel to increase sufficiently so that the metal level in the forehearth decreases below that of the forehearth outlet and thereby enables safe changeover of launders and torpedo cars. After changeover is completed the pressure can be increased as required.
The pressure adjustment step (ii) may include adjusting the pressure to the lower pressure P2 over the whole of the period of time to the next slag tap. Alternatively, the pressure adjustment step may be completed a period of time before the next slag tap and the pressure held at the lower pressure P2 until the next tap.
The tap to tap period will vary depending on the range of factors, such as the size of the vessel and the injection rates and composition of feed materials.
Typically the period of time between slag taps is 2-3 hours.
Preferably the pressure increase steps and the pressure decrease steps in steps (i) and (ii) are 0.5-2 kPa.
More preferably the pressure increase steps and the pressure decrease steps in steps (i) and (ii) are 0.5-1.5 kPa.
Preferably step (b) includes injecting metalliferous feed material, solid carbonaceous material, and fluxes into the metal layer via a plurality of lances/tuyeres.
More preferably the solid carbonaceous material is coal.
Preferably step (c) includes smelting the metalliferous feed material to molten metal in the metal layer.
Preferably the direct smelting process includes causing molten material to be projected as splashes, droplets, and streams into a space above a normal quiescent surface of the molten bath and forming a transition zone.
More preferably the process includes injecting the oxygen-containing gas into the direct smelting vessel via one or more than one lance/tuyere and post-combusting reaction gases released from the molten bath, whereby the ascending and thereafter descending splashes, droplets, and streams of molten material in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via a side wall of the vessel that is in contact with the transition zone.
The term xe2x80x9cquiescent surfacexe2x80x9d in the context of the molten bath is understood to mean the surface of the molten bath under process conditions in which there is no gas/solids injection and therefore no bath agitation.
Preferably the oxygen-containing gas is air or oxygen-enriched air.
More preferably the oxygen-enriched air contains less than 50% by volume oxygen.
Preferably the process operates at high post-combustion levels.
Preferably the post-combustion levels are greater than 60%.
The metalliferous feed material may be any suitable iron-containing feed material. The preferred feed material is iron ore.
The iron ore may be pre-heated.
The iron ore may be partially reduced.